1. Field of the Invention
The present invention relates to an encryption device and method which encrypt sentences or the like, and to an image forming apparatus.
2. Description of the Related Art
In these days, electronization of information is being advanced and duplicable electronized information is easily encrypted by making use of various encryption methods in order to protect the electronized information. For example, in an image forming apparatus such as a printing machine capable of forming an image by receiving a printing command from a personal computer (PC), information stored therein and information to be output are encrypted.
A conventional image forming apparatus receives, for example, from a PC, a command to print a document 70a (FIG. 7A) as a secret document including an encryption-unneeded portion 71 and an encryption-needed portion 72, and encrypts only the encryption-needed portion 72 in the document 70a into an encrypted portion such as an encrypted portion 73 in a document 70b (FIG. 7B). The encryption-unneeded portion 71 is a portion through which the contents of the document 70 can be roughly understood. When the document 70b is received, the document 70b is read by using an image reading apparatus such as a scanner and is converted into character data by character recognition processing. Thereafter, the document 70b can be converted into the document 70a by decrypting and deciphering the encrypted portion 73.
The above-described encryption method ordinarily uses a random number table (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 11-41383). This method encrypts a document by converting it into an amphibology random sequence of characters, as the encrypted portion 73, on the basis of a conversion code table prepared by utilizing the randomness of the random number table. The encrypted document is read by an image reading apparatus in which the conversion code table is stored, to thereby be decrypted into the not-yet encrypted document.
However, the above-described conventional encryption method encrypts and decrypts a document on the basis of a conversion code table; therefore, if the conversion code table is duplicated by a third person, the encrypted document can be easily decrypted and deciphered by the third person.
The conversion code table can be easily duplicated independently of whether or not it is electronized information or printed information. After duplication of the conversion code table, no trances of duplication are left. Therefore, the person having the document in his/her possession cannot infer the possibility that the conversion code table for the document is duplicated and information of the document is leaked.
The conversion code table should be stored both in the image forming apparatus that encrypts the document and in the image reading apparatus that decrypts the document. Thus, the conversion code tables exist in a plurality of places, thereby increasing the risk of the conversion code table being accessed by a third person.